Characteristics And Formation Mechanism Of Foreland Deformation In The Subei Region

The Subei region south of the Sulu Orogen and east of the Tan-Lu Fault Zone is aspecial area near the two tectonic belts and with abundant oil and gas resources.Owing to large-scale cover by the Subei Basin, features and formation mechanismof foreland structures in the Subei region remain unclear. More understanding ofthe foreland structures is of great significance for revealing its tectonic evolutionand exploration of marine oil and gas resources.By analyses of many exploration data from the Subei Basin and observation ofmarine cover exposures, it is suggested that the marine cover in the Subei regionexperienced foreland deformation during the Indosinian period at the end of MiddleTriassic, and regional detachment appears between the cover and basement in theforeland fold and thrust belt. Comparison for a series of outcrop sections in thesouthern edge of the Subei Basin show that the foreland deformation intensityshown by faults and folds from weaken eastwards and tectonic lines change fromNE-strikes, parallel to the Tan-Lu fault zone, to nearly east-west strikes eastwards.According to interpretation for six main seismic sections and many localthree-dimensional seismic sections in the Subei Basin, in combination with drillingand non seismic data, details of the foreland deformation characteristics are presentin this thesis. From northwest to southeast, the styles, sizes, density and intensityof the foreland thrusts and folds in the marine cover show regular changes.Strikes of thrusts and folds in the marine cover show a arc-like change from NNEto ENE from west to east, On the basis of the above deformation characteristics andchange rules, combined with the regional tectonic evolution, the forelanddeformation belt is divided into a root belt, middle belt and front belt from NW toSE. The root belt can be further divided into the Lianshui-Binhai nappe andXuyu-Hongze nappe. The middle belt can be further divided into the southernBaoying nappe, Sheyang nappe, the northern Yancheng nappe, Jianhu-Dafengnappe, Xiaohai nappe, Tianchang-Xinghua nappe and Jiangdu-Dongtai nappe. Thefront belt is expressed as opposite-thrusts affected by both the northern andsouthern foreland belts.Structural features and denudation degrees of the marine cove imply that the foreland deformation in the Subei region increase toward the Sulu Orogen andTan-Lu fault zone and decrease toward the opposite directions. So, the southeastarea of the Subei Basin belongs to relatively stable area, which maybe favorable forexploration of the marine oil and gas. Characteristics and changes of the forelandstructures demonstrate that the foreland deformation results from collision of theSulu Orogen, and is synchronous with continental, deep subduction in the orogen.The arc-like change of the structural strikes and westward increase of thedeformation intensity are caused by sinistral faulting along the Tan-Lu Fault Zoneduring the foreland deformation rather than later faulting. The forelanddeformation features support the view that the Tan-Lu Fault zone experiencedlarge-scale sinistral faulting during the orogenic period.